When you say "size", do you mean occupancy? If so, that would depend on your business plan. I would think that, from a consumer standpoint, a theatre of less than 100-150 seats would start to get into the "I can do this at home" realm.

As for parking, that's going to be determined by the local building and planning department of the municipality in which the theatre is located. Generally, if parking is required, it is some number of spots per some portion of the occupancy.

"screening room" ... if it's a small theatre for screening purposes only... I have seen luxurious theatres with 12 seats and work a day theatres with 50-60. Many plexes now have a 50-60 seat theatre though they usually seem like they could put in more seats. Can you be more specific?